What's more likely to cause a patchy mustache, folliculitis or iron deficiency?

Probably just you. Genetics; some guys can grow a great 'stache, others can't. The more chest hair & thicker beard, the sooner you'll probably go bald. You may also be iron deficient -- get the cause found and treated; hope you're not a non-supplementing vegetarian. Most guys get occasional inflamed follicles; if you have pityrospirum folliculitis, it's easy to treat. Many women prefer sparse facial hair in the us.